The finding of rare fetal nucleated red blood cells (fnRBCS) in the maternal circulation has led to the idea of developing prenatal genetic screening using peripheral venipuncture. The motivations for developing this a minimally invasive method for obtaining fetal cells for genetic testing are to reduce cost and risk, and increase the availability of the procedure. Genetic anomalies are to be detected by ultrasound, amniocentesis and chorionic villus sampling. Although ultrasound is non- invasive, many genetic abnormalities do not result in detectable morphological abnormalities. Thus, ultrasound is combined with amniocentesis or chronic villus sampling (depending on gestational age) for screening at-risk pregnancies. These techniques involve inserting a needle into the uterus and entail risks to the fetus that include a small chance of miscarriage. A simple blood test would represent a low-risk technique with tremendous potential for use in widespread screening. The challenge in utilizing peripheral maternal blood is locating the estimated 1/10,000,000 fnRBCs. Thus far, flow cytometry and other sorting methods have failed to achieve the accuracy and throughput necessary for this magnitude of rare event cell selection. In addition, all currently proposed methods apply enrichment steps to the nRBC- containing buffy coat. These enrichment steps further reduce the number of fnRBCs. The acceptable level of fnRBC loss in enrichment is unknown because the direct measurements of the numbers (and variations) in maternal peripheral blood have not been possible. We propose to further develop image-based cell analysis technology. We propose to further develop image-based cell analysis technology (scanning cytometry) to: 1) measure directly the number of fetal fnRBCs residing in the maternal buffy coat, and 2) evaluate the technology as a routine cell selection method for prenatal genetic screening. Two strategies for scanning cytometry development- a slower, technologically low-risk method and a more untested high-speed continuous-scanning method-are proposed for further development of image-based in situ sorting. The proposed high-performance scanning cytometry for large-scale rare event detection has potential for extremely broad clinical and research use. Revisions (in new font): The primary critiques and revisions are biological (the instrumentation portion received excellent reviews). Dr. Karen Arden, an medical genetics and FISH expert, was added as an investigator to lead the combining of our previously perfected DAPI/anti- HbF-FITC dual stain with a dual X and Y chromosome FISH stain for a 4-color fluorescence technique. Full statistical analysis of the distribution of fnRBCs vs. gestation age will also be performed, but a proposed work will, however, provide the first direct measurement of the numbers of fnRBCs using an instrument with demonstrated high ultra-rate event accuracy on an appropriate number of patients for this first study.